1. Field of the Invention
The present invention relates to a method and to an apparatus for making a hydrate slurry, and relates to a thermal storage method, a thermal storage apparatus, and a thermal storage medium.
2. Description of the Related Art
Some sorts of the storing heat methods have been known as follows.
(1) Thermal Storage by Chilled Water
In air conditioning, chilled water having a temperature of 5 to 7xc2x0 C. is stored in a thermal storage tank when the load on an air conditioner is low. Since chilled water has a specific heat of 1 kcal/kgK, the cooling potential is merely 7 kcal/kg per 1 kg of chilled water when the usable temperature difference is 7xc2x0 C. Thus, in this method, a small amount of thermal storage is disadvantageous.
(2) Thermal Storage by Heat of Fusion of Ice, etc.
Since ice has a heat of fusion of approximately 80 kcal/kg, thermal storage by partial conversion of chilled water to ice has a larger thermal storage density. For example, chilled water containing 20 percent ice by volume has a thermal storage density of approximately 28 kcal/kg including the sensible heat of chilled water when the usable temperature difference is 7xc2x0 C.
Since the chilled water is cooled to a temperature of 0xc2x0 C. or less in this method, the refrigerating machine must have higher power compared to thermal storage by chilled water.
(3) Thermal Storage by Materials Other than Ice
Conventional thermal storage media other than water and ice are inorganic hydrated salts, such as LiClO2.3H2O and Na2SO4.10H2O+NH4Cl, and gaseous hydrates (see xe2x80x9cApplication of Gaseous Hydrates to Cooling Potential Storage Mediaxe2x80x9d (Document 1) by Kawasaki and Akiya; Chemical Engineering, 27(8), 603-608 (1982), and xe2x80x9cNew Energy Technological System for Environmental and Energy Conservationxe2x80x9d (Document 2); p. 802, edited by The Heat Transfer Society of Japan).
These inorganic hydrated salts have relatively large latent heats. These salts, however, do not have congruent melting points (described later), and thus the compositions of the hydrates change with the concentrations of the anhydrous salts. As a result, phase separation will occur in cooling-heating cycles and required thermal storage efficiency will not be achieved.
The gaseous hydrates disclosed in Document 1 are materials having large ozone depletion factors, for example, R11 and R12. Since R12 are present as gas under atmospheric pressure, the thermal storage apparatus requires high-pressure hermetically sealed vessels and tubes, incurring higher facility costs.
Various thermal storage apparatuses used in air conditioners have been developed based on the above-mentioned known thermal storage methods. Such thermal storage apparatuses contribute to effective use of energy. For example, off-peak power in the midnight and variable-output forms of energy, such as exhaust heat from factories, are accumulated as a cooling potential, and the accumulated cooling potential is used in air conditioners.
A typical thermal storage apparatus uses ice. Ice is produced using off-peak power in the midnight and the cooling potential accumulated in the ice is used in an air conditioner during the daytime. As described above, although ice can store a larger amount of cooling potential than water, this apparatus forms a solid ice. By that reason, it needs for a coil for producing an ice. As a result, the air conditioner is inevitably complicated and large. Furtermore, in such a thermal storage apparatus, an ice exists as a solid. Therefore, it is difficult to transport a solid ice. Since the stored ice cannot be directly fed into a heat exchanger of the air conditioner, heat exchange is performed from the stored ice to brine, which is then fed into the air conditioner. Thus, the air conditioner requires additional equipment, increasing costs. In another proposed method, the formed ice is crushed and mixed with water, and the resulting slurry is fed into the air conditioner. The slurry, however, is not maintained in a stable and constant granular distribution, because the melting point of the pulverized ice and the freezing point of water are 0xc2x0 C. In this point of view, the refrigerating machine needs for a high power. In some cases, there occurs a clogging by coagulation under floating.
Some thermal storage apparatuses use hydrates. Water molecules form a cage structure. Other molecules, that is, guest molecules, are included in the cage structure of host molecules to form clathrate hydrates. The hydrates have the appearance and physical properties which resemble those of ice. The temperatures for forming the hydrates change with the type and concentration of the guest molecules and other conditions. Some hydrates are formed at temperatures above the freezing point of water.
Thus, an aqueous slurry including hydrate particles can be formed at a temperature higher than the freezing point of water by selecting the type of the guest molecule and other conditions. The hydrate slurry has a large thermal storage capacity due to a large latent heat of the hydrate, can be easily transferred via a pipe, and facilitates heat exchange. Such a hydrate slurry can be used in a conventional air conditioner using chilled water with minor modifications.
However, in actual use, the hydrate is not produced when the solidification temperature is reached. When the hydrate is cooled under the solidification temperature, the hydrate begins to be produced. This is called the supercooling phenomenon. In the case that the supercooling rate, which means the solidification temperaturexe2x80x94the temperature just before the hydrate is produced is large, it is necessary to lower the refrigerant temperature. Because of that reason, in order to utilize the thermal storage apparatus, which uses a hydrate slurry, it is necessary to decrease said supercooling rate.
Accordingly, it is an object of the present invention to provide a thermal storage method, a thermal storage apparatus and a thermal storage medium for making a hydrate slurry, which have a high fluidity, a latent heat at 0-120xc2x0 C., which will not result in coagulation of hydrate particles, and which will result in a small amount of super-cooling rate. A xe2x80x9chydratexe2x80x9d is one sort of a clathrate hydrate. Hereinafter, xe2x80x9chydratexe2x80x9d has the same meaning as xe2x80x9cClathrate Hydratexe2x80x9d.
It is another object of the present invention to provide a thermal storage method, a thermal storage apparatus, and a thermal storage medium using a clathrate hydrate which has a large thermal storage density (latent heat), stable thermal characteristics, which is economical and easy to handle, and which is safe.
It is another object of the present invention to provide an air conditioner having a thermal storage apparatus having a large thermal storage capacity, a simplified configuration, and which may be compact.
In order to attain the above-mentioned object, this present invention provides a method for producing hydrant slurry, the apparatus thereof and the product thereof. That is to say;
(a) A method in accordance with the present invention, for forming a hydrate slurry by cooling an aqueous solution containing a guest compound to form hydrate particles in the aqueous solution, comprises a preparing step, a cooling step of the aqueous solution being circulated by a heat transfer face, and a contacting step for bringing the nuclear particles contact with the circulated aqueous solution to form the hydrate particles.
The circulated aqueous solution is cooled on a heat transfer face. When the cooled aqueous solution comes into contact with nuclear particles, supercooling of the aqueous solution will not occur and fine hydrate particles are easily formed in the aqueous solution. Thus, the resulting slurry has high fluidity. Fractions of the circulated aqueous solution sequentially contact with the heat transfer face and are supercooled. When the supercooled aqueous solution comes in contact with nuclear particles, hydrate particles form and the supercooled state of the aqueous solution will disappear. That is, the locally supercooled state of the aqueous solution disappears by the formation of the hydrate slurry so that the overall aqueous solution is not supercooled. The hydrate particles have high fluidity.
(b) An apparatus in accordance with the present invention, for making a hydrate slurry by cooling an aqueous solution containing a guest compound to for hydrate particles in the aqueous solution, comprises a heat exchanger having a heat transfer face for cooling the aqueous solution, the aqueous solution being circulated and cooled by contacting with the heat transfer face, and a nuclear particle-supply mechanism for supplying nuclear particles to the aqueous solution circulating in the heat exchanger. This apparatus has a simplified configuration, and any conventional apparatus can be used as this apparatus with minor modifications and minimized additional expense.
(c) An apparatus in accordance with the present invention, for making a hydrate slurry by cooling an aqueous solution containing a guest compound to form hydrate particles in the aqueous solution, comprises a heat exchanger having a heat transfer face for cooling the aqueous solution, the aqueous solution being circulated and cooled by contact with the heat transfer face, and a fine particle layer adhered to at least a part of a surface of a member in the heat exchanger in contact with the aqueous solution, the fine particle layer acting as nuclei of the hydrate particles.
The fine particle layer can prevent supercooling of the aqueous solution and does not cause malfunction of the apparatus because the fine particle layer does not float in the aqueous solution. Furthermore, the apparatus can be operated without recovery and supplement of the fine particles.
(d) An apparatus in accordance with the present invention for making the hydrate slurry which comprises:
a means for cooling an aqueous solution containing a material for forming a clathrate compound hydrate as a guest compound to form hydrate particles;
a heat exchange means for performing heat exchange between a refrigerating machine and an aqueous solution to cool the aqueous solution; and
a circulation means for circulating the aqueous solution through the heat exchange means.
(e) A method in accordance with the present invention for thermal storage using a clathrate hydrate comprises
preparing an aqueous solution containing a material for forming the clathrate hydrate so that the aqueous solution has a concentration of the material which causes a congruent melting point; and
cooling the aqueous solution to form the clathrate hydrate.
achieving a thermal storage, by making use of the clathrate hydrant.
(f) A method for thermal storage using a clathrate hydrate comprises:
preparing an aqueous solution containing a material for forming the clathrate hydrate so that the aqueous solution has a concentration lower than the concentration causing a congruent melting point;
cooling the aqueous solution to form the clathrate hydrate; and
achieving a thermal storage, by making use of the clathrate hydrant.
(g) A product in accordance with the present invention is a thermal storage medium, which is an aqueous solution containing a material for forming a clathrate hydrate.
(h) An apparatus in accordance with the present invention is an air conditioner, which comprises
a refrigerating machine; and
a thermal storage apparatus, connected to the refrigerating machine by piping, for storing a guest compound solution forming a hydrate at a temperature higher than 0xc2x0 C.;
the thermal storage apparatus comprising a heat exchanger for cooling the aqueous solution by a thermal medium from the refrigerating machine to form a slurry of hydrate particles; and
the thermal storage comprising a circulator for supplying the slurry to a load-side device of the air conditioner.